The origin of the use of nanoceria in nanomedicine can be traced to the seminal work of Bailey and Rzigalinski, wherein the application of ultrafine cerium oxide particles to brain cells in culture was observed to greatly enhanced cell survivability, as described by Rzigalinski in Nanoparticles and Cell Longevity, Technology in Cancer Research & Treatment 4(6), 651-659 (2005). More particularly, rat brain cell cultures in vitro were shown to survive approximately 3-4 times longer when treated with 2-10 nanometer (nm) sized cerium oxide nanoparticles synthesized by a reverse micelle micro emulsion technique, as disclosed by Rzigalinski et al. in U.S. Pat. No. 7,534,453, filed Sep. 4, 2003.
Subsequently, a host of problems with these particular nanoceria particles was disclosed by Rzigalinski et al. in WO 2007/002662. Nanoceria produced by the reverse micelle micro emulsion technique suffered as follows: (1) particle size was not well-controlled within the reported 2-10 nanometer (nm) range, making variability between batches high; (2) tailing of surfactants, such as sodium bis(ethylhexyl)sulphosuccinate, also known as docusate sodium or (AOT), used in the process into the final product caused toxic responses; (3) inability to control the amount of surfactant tailing posed problems with agglomeration when these nanoparticles were placed in biological media, resulting in reduced efficacy and deliverability; and (4) instability of the valence state of cerium (+3/+4) over time. Thus, the cerium oxide nanoparticles produced by the reverse micelle micro emulsion technique were highly variable from batch to batch, and showed higher than desired toxicity to mammalian cells.
As an alternative, Rzigalinski et al. in WO 2007/002662 describe the biological efficacy of nanoceria synthesized by high temperature techniques, obtained from at least three commercial sources. These new sources of cerium oxide nanoparticles were reported to provide superior reproducibility of activity from batch to batch. It was further reported that, regardless of source, cerium oxide particles having a small size, narrow size distribution, and low agglomeration rate are most advantageous. In regard to size, this disclosure specifically teaches that in embodiments where particles are taken into the interior of cells, the preferable size range of particles that are taken into the cell are from about 11 nm to about 50 nm, such as about 20 nm. In embodiments where particles exert their effects on cells from outside the cells, the preferable size range of these extracellular particles is from about 11 nm to about 500 nm.
These inventors (Rzigalinski et al.) also report that for delivery (administration), the nanoparticles were advantageously in a non-agglomerated form. To accomplish this, they reported that stock solutions of about 10% by weight could be sonicated in ultra-high purity water or in normal saline prepared with ultra-high purity water. However, we have confirmed what others have observed, that sonicated aqueous dispersions of nanoceria (synthesized by high temperature techniques and obtained from commercial sources) are highly unstable, and settle rapidly (i.e. within minutes), causing substantial variability in administering aqueous dispersions of nanoceria derived from these sources.
Yokel et al. in Nanotoxicology, 2009, 3(3): 234-248, describe an extensive study of the biodistribution and oxidative stress effects of a commercial ceria nanomaterial. In particular, a 5% nanoceria dispersion obtained from Aldrich (#639648) was sonicated for 3 minutes and infused into rats at 50, 250 and 750 mg/kg nanoceria dose. The nature of any nanoparticle surface stabilizer(s) was unknown for this material. The size of the nanoceria particles was characterized by a variety of techniques and reported to be on average 31+/−4 nm by dynamic light scattering. Transmission electron microscopy (TEM) revealed that most of the particles were platelets with a bimodal size distribution with peaks at 8 nm and 24 nm, along with some particles ˜100 nm. It was observed that blood incubated for 1 hour with this form of nanoceria had agglomerates ranging from ˜200 nm to greater than 1 micron, and that when infused into rats, it was rapidly cleared from the blood (half-life of 7.5 minutes). Most of the nanoceria was observed to accumulate in the liver and spleen, while it was not clear that any substantial amount had penetrated the blood brain barrier and entered brain tissue cells.
This group of authors then sought precise control over the nanoceria surface coating (stabilizer) and prepared stable aqueous dispersions of nanoceria by the direct two-step hydrothermal preparation of Masui et al., J. Mater. Sci. Lett. 21, 489-491 (2002), which included sodium citrate as a biocompatible stabilizer. High resolution TEM revealed that this form of nanoceria possessed crystalline polyhedral particle morphology with sharp edges and a narrow size distribution of 4-6 nm. Citrate stabilized dispersions of these 5 nm average ceria nanoparticles were reported to be stable for more than 2 months at a physiological pH of 7.35 and zeta potential of −53 mV. Thus no sonication prior to administration was required.
Results of an extensive biodistribution and toxicology study of this form of citrate stabilized nanoceria was reported by this group of authors in Hardas et al., Toxicological Sciences 116(2), 562-576 (2010). Surprisingly, they report that compared with the previously studied ˜30 nm nanoceria (Aldrich (#639648) described above), this nanoceria was more toxic, was not seen in the brain, and produced little oxidative stress effect to the hippocampus and cerebellum. The results were contrary to the hypothesis that smaller engineered nanomaterial would readily permeate the blood brain barrier.
While cerium oxide containing nanoparticles can be prepared by a variety of techniques known in the art, the particles typically require a stabilizer to prevent undesirable agglomeration of the nanoparticles. In regard to biocompatible nanoceria stabilizers used previously, once again, Masui et al., J. Mater. Sci. Lett. 21, 489-491 (2002) describe a two-step hydrothermal process that directly produces stable aqueous dispersions of ceria nanoparticles that use citrate buffer as a stabilizer. However, this process is both time and equipment intensive, requiring two separate 24 hour reaction steps in closed reactors.
Sandford et al., WO 2008/002323 A2, describe an aqueous preparation technique using biocompatible acetic acid stabilizer that directly produces nanoparticle dispersions of cerium dioxide without precipitation and subsequent calcination. Cerous ion is slowly oxidized to ceric ion by nitrate ion, and a stable non-agglomerated sol of 11 nm crystallite size (and approximately equal grain size) is obtained when acetic acid is used as a stabilizer.
DiFrancesco et al. in commonly assigned PCT/US2007/077545, METHOD OF PREPARING CERIUM DIOXIDE NANOPARTICLES, filed Sep. 4, 2007, describes the oxidation of cerous ion by hydrogen peroxide at low pH (<4.5) in the presence of biocompatible alpha-hydroxy carboxylic acid stabilizers, such as lactic acid, tartaric acid, gluconic acid and 2-hydroxybutanoic acid. Specifically, the stabilizer lactic acid and the stabilizer combination of lactic acid and ethylenediaminetetraacetic acid (EDTA) are shown in working examples to directly produce stable dispersions of nanoceria particles of average particle size in the range of 3-8 nm under highly acidic reaction conditions.
Karakoti et al. in J. Phys. Chem. C 111, 17232-17240 (2007) describe the direct synthesis of nanoceria in mono/polysaccharides by oxidation of cerous ion in both acidic conditions (by hydrogen peroxide) and basic conditions (by ammonium hydroxide). The specific biocompatible stabilizers disclosed include glucose and dextran. Individual particle sizes as small as 3-5 nm are disclosed, however, weak agglomerates of 10-30 nm resulted. While the source of the colloidal instability is not described, we speculate that the magnitude of the zeta potential of these particles may not have been sufficiently large.
Karakoti et al. in JOM (Journal of the Minerals, Metals & Materials Society) 60(3), 33-37 (2008) comment on the challenge of synthesizing stable dispersions of nanoceria in biologically relevant media, so as to be compatible with organism physiology, as requiring an understanding of colloidal chemistry (zeta potential, particle size, dispersant, pH of solution, etc.) so as not to interfere with the reduction/oxidation (redox) ability of the nanoceria that enables the scavenging of free radicals (reactive oxygen species (ROS) and reactive nitrogen species). These authors specifically describe the oxidation of cerium nitrate by hydrogen peroxide at low pH (<3.5) in the absence of any stabilizer, as well as, in the presence of dextran, ethylene glycol and polyethylene glycol (PEG) stabilizers. Particle sizes of 3-5 nm are reported, although particle agglomeration to 10-20 nm is also disclosed.
As described above, various methods and apparatus have been reported for preparing dispersions of nanoceria particles. However, a need remains for further improvements in methods for the direct preparation of biocompatible dispersions of nanoceria particles, for example, without isolation and subsequent dispersal of the nanoparticles, and without a calcination step to impart crystallinity, in higher yield, in a shorter period of time and at higher suspension densities, that are sufficiently small in size (e.g. sufficiently small in size to evade detection by an immune system), uniform in size frequency distribution, stable and non-toxic in a wide range of biological media. In addition, it would be desirable to produce dispersions of these nanoparticles that are compatible with physiological pH conditions.